The invention relates to a method of manufacturing a semiconductor device comprising a semiconductor body with a bipolar transistor including a base, an emitter and a collector, said base being formed by providing the semiconductor body with a doped semiconducting layer (1) which locally borders on a monocrystalline part of the semiconductor body (10), thereby forming a first semiconductor region which is monocrystalline and which constitutes the base of the transistor, and which semiconducting layer borders, at locations outside said base, on a non-monocrystalline part of the semiconductor body, thereby forming a second semiconductor region which is not monocrystalline and which constitutes a connection region of the base, and, after the provision of the semiconducting layer, this layer being provided, above the base, with a mask, whereafter, outside said mask, the doping concentration of the semiconducting layer is selectively increased. Such a method enables very fast transistors to be manufactured in a simple manner.
Such a method is known from United States patent specification U.S. Pat. No. 5,117,271, published on May 26, 1992. In said document, a description is given of a method in which a mesa is formed in a semiconductor body, which mesa is the collector of the transistor to be formed. The mesa is formed by making grooves in a monocrystalline silicon substrate provided with an epitaxial layer. Thus, the mesa forms a monocrystalline part of the semiconductor body. The grooves are filled with an insulating oxide and hence constitute a non-monocrystalline part of the semiconductor body. By means of a growth process, subsequently, a doped semiconductive layer of silicon is provided on both parts of the semiconductor body. Above the mesa, this layer forms a monocrystalline base, and above the insulating oxide, the layer forms a non-monocrystalline, in this case polycrystalline, region which serves as the connecting region of the base. Such a method, in which the base is manufactured by means of a growth process, enables very fast bipolar transistors to be manufactured. This can be attributed to the fact that a base produced by means of a growth process can be readily provided, in the course of the growth process, with an optimum doping profile for the speed of the transistor. The same applies to the provision of a heterojunction, for example a Si--SiGe heterojunction, by means of which the maximum speed of the transistor can be increased too. Also a possible graduation of the composition of a SiGe part of the base enables even higher speeds to be attained, because the connection losses are reduced. To maximize the speed of the transistor, it is desirable, however, for the resistance of the connecting region to be as low as possible. To achieve this, the grown semiconductive layer is locally masked above the base, and, outside the mask, the semiconductive layer is provided with a higher doping concentration by means of an ion implantation. The transistor is completed by diffusing an emitter in (a part of) the base.
A drawback of the known method is that the resultant transistors are not as fast as desired.